The present disclosure relates to a bone anchoring device.
A bone anchoring device comprising a shank to be anchored in a bone and a receiving portion to connect the shank with a rod, wherein the receiving portion and the shank are formed as a single piece is known, for example, from U.S. Pat. No. 5,005,562 or from DE 101 57 969 C1. At the time of screwing in such a so-called monoaxial bone screw the receiving portion is aligned to receive the rod by rotating the shank within its fixation in the bone. The depth into which the shank can be screwed into the bone, depends on the required orientation of the receiving portion relative to the rod. Therefore, the possibility of fine-adjusting the orientation of the receiving portion relative to the rod depends on the thread pitch of the threaded shank. Usually monoaxial bone screws can not be screwed-in to be fully seated within the anatomical dimensions.
A polyaxial bone anchoring device, wherein the shank and the head are separate parts, is known from U.S. Pat. No. 6,835,196 E2. This bone anchoring device comprises a shank to be anchored in the bone and a head which has an exterior surface with a spherical segment-shaped portion, a receiving portion to receive said head and an element which exerts pressure on said head. The shank is received in a bore in the head which has a spring-yielding edge to clamp said end portion of the shank when pressure is exerted on the head. Since the head has an exterior surface with a spherical segment-shaped portion, the head can be pivoted in the receiving portion as long as a pressure is not exerted onto the head. With this bone anchoring device it is possible to first screw in the shank, adjust the length of the shank, if necessary, and then connect it with the receiving portion containing the head.
For certain clinical applications monoaxial bone screws are more appropriate than polyaxial bone screws. However, the known monoaxial bone screws have the disadvantage that there is only a small possibility of alignment of the receiving portion relative to the rod and yet be fully seated in the bone so as to be flush with the bone surface.
In view of the above, there is a need for a bone anchoring device that can remedy one or more of the above described problems associated with current bone anchoring devices.